Calcination Temperature Dependent Catalytic Activity and Stability of IrO₂–Ta₂O₅Anodes for Oxygen Evolution Reaction in Aqueous Sulfate Electrolytes

Abstract

In this work, commercial IrO 2 -Ta 2 O 5 anodes with a certain composition calcined at three different temperatures were investigated. The results show that the calcination temperature has a significant influence on the electrocatalytic activity for the oxygen evolution reaction (OER). This is attributed to the influence of the calcination temperature on the surface microstructure including the crystallinity and the preferred orientation of IrO 2 crystallites of the IrO 2 -Ta 2 O 5 binary oxide formed. The surface morphology of the anodes was revealed as mud-cracks surrounded by flat areas containing several scattered IrO 2 nanocrystallites. The size of these nanocrystallites, which in turn contribute to the electrochemical active surface area, is dependent on calcination temperature. The (101)-surfaces of the IrO 2 were found to have higher catalytic activity than (110) IrO 2 with respect to the OER. The (101) IrO 2 planes were dominating at low or moderate calcination temperatures, whereas the (110) IrO 2 orientation was preferred at the highest calcination temperature. Accelerated lifetime tests of the investigated samples indicate that the (101) IrO 2 is more stable (110) IrO 2 during electrolysis. A moderate temperature is suggested as the best calcination temperature for this type of anode regarding the electrochemical active surface area, electrocatalytic activity and stability for OER in acidic aqueous electrolytes at operating conditions. © The Author Efficient electrowinning (EW) in aqueous sulfate electrolytes depends on fast reaction kinetics, low ohmic resistances and suppression of parasitic and detrimental reactions. The overall cell voltage is determined by the thermodynamic potentials for metal deposition (cathode) and oxygen evolution (anode), in addition to overpotentials and ohmic voltage drops. The sluggish reaction kinetics of the oxygen evolution reaction (OER) in low-pH sulfate electrolytes lead to rather high anode overpotential at industrial relevant current densities, thus being a significant contributor to an increased cell voltage.1 The low pH, moderate temperature and high anode potential in aqueous metal electrowinning limit the anode material selection significantly, as few materials are stable at these operating conditions. Therefore, identifying an efficient anode catalyst to facilitate the OER by lowering the overpotential has been considered an important research field over many decades also in copper EW.2,3 From an industrial perspective, stability and service lifetime of the anodes are just as important as the electrocatalytic activity. Ru oxide catalysts are known to be the most active for OER, 4 but not stable enough for long term operation in the acidic environment.5 IrO 2 is also very active toward OER and significantly more stable than RuO 2 , but also suffers from some degradation during prolonged operation. [6][7][8][9] Comninellis and Vercesi performed a comprehensive study of nine different binary catalyst coatings.10 They reported that the 70 mol% IrO 2 -30 mol% Ta...

Efficient OER Catalyst with Low Ir Volume Density Obtained by Homogeneous Deposition of Iridium Oxide Nanoparticles on Macroporous Antimony‐Doped Tin Oxide Support

“…The overall high Ir mass‐based activity of the synthesized iridium oxide catalyst is attributed to the small size of the spherical IrO 2 nanoparticles obtained by the synthesis approach and the low‐temperature calcination that is known to result in an increased fraction of high‐surface‐energy crystal facets and defects which are associated with high OER activity . However, the key factor for increased OER activity is the high Ir volume dispersion resulting from the homogeneous coating of nanoparticles onto the surface of the ordered porous conducting support.…”

The durability of the thermally decomposed IrO2-Ta2O5 coated titanium anode in a sulfate solution

“…Apparently the increase of calcination temperature will increase the crystallinity of the IrO 2 phase of the coating. Moreover, there is no Ta 2 O 5 related reflection being captured since the Ta 2 O 5 phase is amorphous at the current calcination temperature range [19]. Here, it should be added that the coating cannot be fully oxidized in this case, even at T4.…”

“…All experiments were conducted in 0.9 M H 2 SO 4 aqueous electrolyte at 60 • C in a temperature controlled water bath. Preconditioning is necessary as being discussed in our previous work [19], and thus all anodes were run at 100 mV s −1 with 200 potential cycles to give satisfactory reproducibility before recording the reported E-I curves. Besides, the electrolyte was de-aerated by argon gas for 30 minutes prior to all measurements.…”

“…Compared to standard PDF card of rutile IrO 2 phase, the observed IrO 2 peaks are slightly shifted to higher 2θ direction with an increase of calcination temperature. This could be because of the formation of Ir(Ta)O 2 solid solution which induces lattice strain into the coating layer [19].…”

Effect of Adding Polyethylene Glycol to the Precursor Solution of Amorphous IrO₂-Ta₂O₅ Electrocatalysts for Oxygen Evolution Reaction

“…17 Large surface area of the substrate is beneficial in increasing the ECSA and the contact area between the electrocatalytic and the substrate layers. 18,19 High-temperature calcination usually forms rutile-type IrO 2 crystallites, 20 but the IrO 2 coated on Ti felt was amorphous even after calcination at 650°C. 17 Amorphous IrO 2 has been reported to exhibit high OER activity compared to crystalline IrO 2 .…”